The present invention relates to a method and apparatus for removing condensable components from hydrocarbon gas; and more particularly, the invention relates to a low temperature fractioning process for removing propane and heavier hydrocarbons from natural gas.
There are a number of processes in the prior art relating to the rejection of methane and ethane from natural gas to recover the heavier fractions, i.e., LPG and the natural gasoline components. These processes, however, are of limited efficiency with only about 65% to 70% of the propane content of the natural gas being recoverable when substantial percentages of the ethane content is rejected.
In a conventional method of recovering propane and the heavier fractions from natural gas, the natural gas is cryogenically processed by refrigerating the gas to a low temperature to effect a greater separation of the propane and heavier components from the natural gas stream. It is, however, expensive to produce refrigeration; and the deeper the refrigeration that is needed, the more expensive it becomes to process the gas. This is particularly so since the increase in expense with respect to the depth of refrigeration is not a linear function, but increases at an increasing rate of change.
To achieve a high efficiency of separation of propane from a natural gas stream, it is necessary to attain a temperature of about -60.degree. F. To produce this low temperature, a refrigeration mechanism is needed utilizing a refrigerant capable of attaining this low temperature. Common refrigerants in use include freon and propane. Freon, though a very desirable refrigerant, is very expensive, and its use in quantities such as would be required in a refrigeration system capable of cooling a large volume of gas to -60.degree. F. would be prohibitively expensive in the application of natural gas processing to recover propane. Propane, though less expensive than freon, is limited in its use to those situations where the temperature need not be taken below about -40.degree. F. In order to utilize propane as the primary refrigerant to attain -60.degree. F., it is necessary to place the propane under a vacuum. This type of operation does, however, present a danger of explosion in the event an air leak develops in the system, as propane in the presence of air can create an explosive mixture.
Accordingly, conventional refrigeration techniques for recovering propane from natural gas have been in most market situations too expensive to use. Also, even where such techniques have been utilized, the propane recovery efficiency has been low, on the order of 75%, with 30%-40% of the ethane content being retained also.
Another process in the prior art for recovering propane from natural gas is that disclosed in U.S. Pat. No. 3,292,380 issued Dec. 20, 1966. In that process, natural gas under pressure is pre-cooled and expanded through a turbine to produce a gas-condensate mixture at a low temperature. This mixture is then fed to a separator where the liquids, LPG and heavier hydrocarbons including the natural gasolines, are separated from the gas, methane and ethane. This process may be carried out to achieve greater recovery of propane, such as about 90%; but there is a concomitant increase in the amount of ethane in the product to the extent of about 35% to 40% of the ethane content in the inlet gas stream.
Another process in the prior art for separating propane from a natural gas stream, and one with which a propane recovery efficiency of approximately 90% with high ethane rejection can be achieved, is that wherein a conventional turbo-expander plant is first utilized to reject the methane content of the natural gas stream, thereby yielding a high ethane and heavier hydrocarbon component recovery from the stream. The ethane and heavier component recovery products are then fed to a de-ethanizer to reject a large portion of the ethane, leaving substantially only the propane and heavier gasoline components.
Although refrigeration equipment is needed in this process, the de-ethanization may be carried out at +60.degree. F. rather than -60.degree. F. But, the expense involved in providing the two stage process is significant. As well as the additional equipment that is required for such a two stage process, i.e., extra tower and extra instrumentation, the maintenance necessary to keep the plant running and avoid costly down time is great.
In addition to the above techniques and processes for separating propane and heavier hydrocarbons from a natural gas stream, yet another low temperature process using a turbo-expander is described in U.S. Pat. No. 2,601,009 issued June 17, 1952. In this process, inlet natural gas is pre-cooled to a temperature below 0.degree. F. and fed to a separator where the condensed liquids accumulate at the bottom of the separator and the gas is brought out of the top of the tower. The gas is passed through an expander to cool the gas, and it is thereafter applied to a reflux condenser the upper end of the separator tower. This step has the effect of condensing the heavier hydrocarbon components to yield greater recovery thereof. Afterward, the de-propanized gas is compressed and supplied to a pipeline. This process, though covering 95% to 97% of the propane content of the inlet natural gas stream, also has in the compressed gas outlet stream approximately 50% of the ethane content of the inlet natural gas.
Although several processes have been proposed in the past for the separation of propane and heavier hydrocarbons from a natural gas stream, the processes have not found widespread utilization due to the significant expense attendant their implementation, or because there is a low propane recovery efficiency where a high rejection of ethane is attained. Accordingly, it would be desirable to have a process for the separation of propane and heavier hydrocarbons from a natural gas stream wherein only a slight percentage of the propane content of the inlet natural gas stream is lost in a separation that rejects substantially all the ethane content of the natural gas stream.